This seems to be very hit and miss. I have noticed people having this issue and some get resolved and some are not. Here is the only trend I noticed...
If you had BBM 5.0 previous to the 5.0 OS upgrd you will likely not have BBM at all or it is not working after the upgrd.
If you had previous BBM is either worked or it didn't, but at least it was present.
Here's what worked for me... I deleted and reinstalled ALL my apps (including BBM) and all downloads were done via App World, but that was the first one I deleted and reinstalled via web browser. (blackberry.com/appworld/download just in case)
I know that isn't much help if you've tried it, but much of this is trial and error not only for me, my coworkers, and my customers. I am updating 2 Storms for a customer at this moment and am crossing my fingers!
I'm having the same problem. When I upgraded it deleted it so I reinstalled it and after I reboot it comes up as archived and then when I try restoring it from archived it disappears again.
